The present invention relates to a process for producing a toner for use in developing an electrostatic latent image in electrophotography, electrostatic printing, electrostatic recording/printing, etc. 2. Description of the Prior Art:
In known copying machines utilizing the principle of electrophotography, a toner in which additives such as a colorant are dispersed in a fixer resin, was used for the purpose of making visible an electrostatic latent image that is formed on a photosensitive plate or drum of selenium or other organic photoconductive material.
According to this known developing method, an electrostatic latent image formed on the photosensitive drum by charging the drum surface and then by exposing the charged drum surface to light is developed by the toner, and then the developed toner image is transferred to a substrate such as a copy paper, whereupon the toner image is fixed to the substrate by a heater roller or a pressure roller. Thus a visible image has been formed on the substrate. After the toner image has been transferred to the substrate, the leavings of the toner particles on the drum surface are cleared off by a cleaning means such as a scraper blade.
The toner is composed of resin particles of 1 to 30 .mu.m particle size in which toner-characteristic imparting agents such as a colorant and a charge controlling agent are dispersed in a fixer resin. The resulting toner is then mixed with a carrier to form a developer.
The carrier is in the form of iron powder or ferromagnetic powder. In development, the resultant developer forms a magnetic brush by the action of a magnet roll of a developing unit. As the magnet roll is rotated, the magnetic brush is brought toward the electrostatic latent image area, where only the toner is adhered to the latent image to develop a visible image. In order to reproduce the original image with fidelity, the individual toner particles are required to have a uniform electric charge so that the image density is kept from lowering due to the decrease of amount of the toner adhered to the latent image, which is caused partly due to the transfer of the uncharged toner particles to the area outside the latent image and partly due to the accumulation of the overcharged toner particles in the developing unit.
At the stage of fixing, the toner is also required to have a delicate heat-melt characteristic and a specific viscosity so that the toner is allowed to be fixed quickly without causing an offset phenomenon, in which as it passes an image-fixing roller, the next substrate is stained with part of the toner adhered to the surface of a fixing roller during the fixing of the toner image on the preceding substrate.
As discussed above, at different stages of the copying process, various characteristics and properties are required within the toner in the form of coloring resin particles in order to assist individually and in combination in forming a clear image.
Further, in recent years, toners are used in copying machines, printers, facsimile machines, and color copying or printing machines. In order to meet these expanding applications and the demand of the color age, a manufacturer must produce various kinds of toners each in a relatively small quantity.
Conventionally, a toner was produced by providing a kneaded product by applying a great shearing force to a mixture of a fixer resin, a colorant, a charge controlling agent, etc. to soften the fixer resin and also to disperse the colorant in the Fixer resin, and then by pulverizing and sizing the kneaded product as cooled.
In this conventional method of production, partly because molecule chains in the fixer resin tend to be broken off due to the great shearing force, and partly because the distribution of molecular weight tends to shift to the low, there is a danger that the resin characteristics would be modified remotely From the requirements. Further, non-uniform dispersion of the colorant and other additives tends to occur due to the varying local molecular weight of the resin, and yet adequate Fixing and developing characteristics, especially For a high-resolution image, can hardly be achieved by adjusting the degree of kneading. Moreover, this melt-kneading requires a great amount of energy and hence a large-scale apparatus, which are cost-consuming. Therefore this conventional method is not suitable for use in manufacturing many kinds of toners each in a small quantity at a high rate of production.
Meanwhile, many proposals were made with the hope to improve the suspension-polymerization method, by which resin particles of the required toner size can be obtained at the stage of polymerization of the fixer resin. In this known method, however, partly because it is a must to use a dispersion stabilizer for stabilizing suspension oil-drop particles in water phase, and partly because the dispersion is adhered to the produced toner particles, the resultant toner is highly dependent on moisture so that deterioration of an image would occur at high humidity.